He started work at Netscape Communications Corporation in April 1995. Eich originally joined intending to put Scheme "in the browser",[6] but his Netscape superiors insisted that the language’s syntax resemble Java’s. The result was a language that had much of the functionality of Scheme, the object orientation of Self, and the syntax of Java. The first version was completed in ten days in order to accommodate the Navigator 2.0 Beta release schedule,[6][7] and was called Mocha, but renamed LiveScript in September 1995 and later JavaScript in the same month.[8] Eich continued to oversee the development of SpiderMonkey, the specific implementation of JavaScript in Navigator.[9]

In early 1998, Eich co-founded the Mozilla project with Jamie Zawinski and others, creating the mozilla.org website, which was meant to manage open-source contributions to the Netscape source code. He served as Mozilla's chief architect.[10]AOL bought Netscape in 1999. After AOL shut down the Netscape browser unit in July 2003, Eich helped spin out the Mozilla Foundation.[11]

In August 2005, after serving as Lead Technologist and as a member of the Board of Directors of the Mozilla Foundation, Eich became CTO of the newly founded Mozilla Corporation, meant to be the Mozilla Foundation's for-profit arm.[11] Eich continued to "own" the Mozilla SpiderMonkey module, its JavaScript engine, until he passed on the ownership of it in 2011.[9]

On March 24, 2014, Eich was promoted to CEO of Mozilla Corporation.[12]Gary Kovacs, John Lilly and Ellen Siminoff resigned from the Mozilla board prior to the appointment,[13] some anonymously expressing disagreements with Eich's strategy and their desire for a CEO with experience in the mobile industry.[14][15] Some employees of Mozilla Foundation (a separate organization from Mozilla Corporation) tweeted calls for his resignation, with reference to his donation of $1,000 to California Proposition 8, which called to ban same-sex marriage in California.[16][17] Eich stood by his decision to fund the campaign, but wrote on his blog that he was sorry for “causing pain” and pledged to promote equality at Mozilla.[13][18] Some of the activists created an online campaign against Eich, with online dating site OkCupid automatically displaying a message to Firefox users with information about Eich's donation, and suggesting that users switch to a different browser (though giving them a link to continue with Firefox).[19][20][21] Others at the Mozilla Corporation spoke out on their blogs in his favor.[22][23] Board members wanted him to stay in the company in a different role.[24] On April 3, 2014, Eich stepped down as CEO and resigned from working at Mozilla; in his personal blog, Eich posted that "under the present circumstances, I cannot be an effective leader."[25][26]

Eich is the CEO of Brave Software, an Internet security company which has raised $2.5 million in early funding from angel investors.[4][27]

On January 20, 2016, the company released developer versions of its open-source Brave web browser, which blocked ads and trackers and included a micropayments system to offer users a choice between viewing selected ads or paying websites not to display them.[28]

At Brave Software, Eich co-created the Basic Attention Token (BAT), a cryptocurrency designed for use in the Brave browser. BAT launched its ICO on May 31, 2017 raising $35 million.[29]